This invention relates to in general to gas solids heat exchangers and especially to coolers for hot particulate materials such as hot cement clinker which is discharged from a furnace such as a rotary kiln. More particularly, the invention relates to a two stage material cooler wherein the first stage serves to initially cool the material as it is discharged from the furnace and serves as a primary source of preheated gas to be supplied to the kiln as secondary air for combustion and the second cooler consists of a shaft type cooler for finally cooling the material.
Prior to the present invention it was generally known to cool particulate material being discharged from a furnace such as a kiln in various apparatus such as an attached tube cooler or a reciprocating grate type cooler both well known in the art. Each of these primary coolers has its advantages and disadvantages. It is also known prior to the present invention to utilize a two stage cooling apparatus wherein a reciprocating grate type cooler is utilized to achieve a rapid quench cooling of the material discharged from the furnace and a secondary cooler is utilized to achieve final cooling of the material. Such an apparatus is shown for example in U.S. Pat. No. 3,705,620 wherein a primary direct heat exchanger is followed by a secondary indirect heat exchanger. An indirect heat exchanger is inherently less efficient than a direct heat exchanger. Such a device is also shown in U.S. Pat. No. 3,824,068 wherein a primary recuperator or first stage heat exchanger is followed by a secondary shaft type cooler wherein product cooling takes place by direct heat exchange between a cooling gas and the material. In each of these prior apparatus, the gas which is collected from the outlet of the second cooling stage can be supplied to the gas inlet of the first cooling stage for use as cooling gas therein.
It is also known prior to the present invention to have a gas to particulate material heat exchanger wherein the material moves generally vertically in a material shaft and the gas passes through the material generally perpendicular thereto through gas permeable sidewalls of the material shaft. Such an apparatus is illustrated in U.S. Pat. No. 3,284,072 as a preheater for solid particulate material to be supplied to a kiln.
The use of a perpendicular gas flow path has the advantage that the cooling gas only passes through a shallow depth of material thereby reducing the pressure drop across the heat exchanger and consequent reduction in energy consumption. While the shallow depth of material means less gas-solids contact at any given point, the height of the material shaft results in the material being exposed to cooling gas for a long period of time to achieve effective cooling. This compares with counterflow shaft type heat exchangers wherein the material flows generally vertically downward through a shaft and the gas moves generally vertically upward through material in the shaft countercurrent to the flow of material. This type of cooler often has a deep bed of material with the resultant advantage of intimate contact between the particulate material and gas to achieve complete heat exchange. The deep bed of material results in the disadvantage of high pressure drop and consequent increase in energy consumption due to the need for a high pressure fan.
Also prior to the present invention as illustrating in U.S. Pat. No. 3,704,525, in an effort to reduce or eliminate the need for a high efficiency dust collector in conjunction with a grate type cooler, it was known to recirculate cooling gas back to the inlet for cooling gas after it had passed through an air to air heat exchanger or other gas cooler thereby eliminating a vent to atmosphere and the need for a high efficiency dust collector.
According to the present invention, a material cooler has been provided which combines the advantages of the various known types of coolers for particulate material such as cement clinker while eliminating some of the disadvantages of such coolers. For example, the present invention can be utilized to achieve a rapid initial cooling of the material discharged from the kiln, i.e. quench cooling of the material. This is particularly advantageous when cooling cement clinker. The present invention combines the advantage of a shaft cooler with the advantage of a crossflow cooler by utilizing a secondary cooler which has cooling gas flow both countercurrent to the flow of material and across to the flow of material. The invention also combines the advantage of a recirculation to eliminate or substantially eliminate the need for high efficiency dust collection equipment for removing fine material carried over with the cooling gas.
The two stage material cooler of the present invention offers further advantages over existing designs. These include low power consumption due to a transfer of a large portion of the cooling process to the secondary cooler that requires only small amounts of power for its material feeders. A second advantage is reduced power requirements for the cooling air fans for the panel bed cooler since the cooling gas is flowing through a relatively thin, vertical bed or panel of material. A further advantage is reduced capital costs for the equipment and elimination or reduction of dust collection equipment requirements.